Process monitoring and control systems are used to monitor and control operation of industrial processes. Industrial processes are used in manufacturing to produce various products such as refined oil, pharmaceuticals, paper, foods, et cetera. In large scale implementations, these processes must be monitored and controlled in order to operate within the desired parameters.
“Transmitter” has become a term which is used to describe the devices which couple to the process equipment and are used to sense a process variable. Example process variables include pressure, temperature, flow, and others. Frequently, a transmitter is located at remote location (i.e. in the “field”), and transmits the sensed process variable back to a centrally located control room. Various techniques are used for transmitting the process variable including both wired and wireless communications. One common wired communication technique uses what is known as a two wire process control loop in which a single pair of wires is used to both carry information as well as provide power to the transmitter. One well established technique for transmitting information is by controlling the current level through the process control loop between 4 mA and 20 mA. The value of the current within the 4-20 mA range can be mapped to corresponding values of the process variable. Other communication protocols include the HART® communication protocol in which a digital signal is modulated on top of a 4-20 mA communication current analog signal, a FOUNDATION™ Fieldbus protocol in which all communication is carried out digitally, wireless protocols such as WirelessHART (IEC 62591), et cetera.
One type of transmitter is a pressure transmitter. In general, a pressure transmitter is any type of transmitter which measures a pressure of a fluid of the process. (The term fluid includes both gas and liquids and their combination.) A pressure transmitter can be used to measure pressures directly including differential, absolute or gage pressures. Further, using known techniques, pressure transmitters can be used to measure flows of the process fluid based upon a pressure differential in the process fluid between two locations.
Typically, a pressure transmitter includes a pressure sensor which couples to the pressure of the process fluid through an isolation system. The isolation system can comprise, for example, an isolation diaphragm which is in physical contact with the process fluid and an isolation fill fluid which extends between the isolation diaphragm and the pressure sensor. The fill fluid generally comprises a substantially incompressible fluid such as oil. As the process fluid exerts a pressure on the isolation diaphragm, changes in the applied pressure are conveyed across the diaphragm, through the isolation fluid and to the pressure sensor. Such isolation systems prevent the delicate components of the pressure sensor from being directly exposed to the process fluid.
A number of commercially-available process fluid pressure transmitters can be used effectively to measure process fluid pressure. These devices generally bring the pressure to the transmitter by virtue of an isolation system or length of pipe filled with process fluid. Examples of such architectures are shown in FIGS. 1A-1D.
FIG. 1A shows a typical steam flow installation. The process fluid pressure transmitter 10 is mounted away from the process 12 due to high temperatures. A pair of pressure impulse lines 14, 16 is used with multiple connections and vents to bring the process pressure to transmitter 10.
FIG. 1B illustrates a high temperature pressure transmitter. Pressure transmitter 20 is mounted away from the process due to high temperatures by using a secondary oil filled system for transporting pressure.
FIG. 1C is a typical remote seal system 30. In this case, the pressure is transported back to transmitter 32 through an oil filled secondary system 34.
FIG. 1D is a diagrammatic view of a flowmeter 40 where a primary element 42 creates a differential pressure. The differential pressure is transported by two impulse lines inside tube 44 up to the coplanar transmitter interface 46.
The architectures illustrated with respect to FIGS. 1A-1D have been successful and offer a number of advantages. The modular transmitter design has enabled high volume production and a highly controlled process to enhance performance. The standard coplanar interface permits distribution efficiencies and a separation point for calibration and replacement. However, these architectures do have some limitations. For example, bringing the pressure to the transmitter is costly as it requires considerable metal and secondary pressurized systems. The architecture may be subject to potential leak points, plugged lines and other impulse line issues. Moreover, these architectures may also be susceptible to mechanical vibration.
It would advance the art of process fluid pressure measurement and control to provide an architecture that can measure the pressure at its source without the need to transport this pressure outside of the normal process pressure boundaries.